Understanding Straightness in GD&T and Methods of Measurement

Geometric Dimensioning and Tolerancing (GD&T) is a standardized system design technique engineers use to communicate tolerance requirements, design intent, and quality procedures to production personnel.
The straightness symbol is a highly practical GD&T symbol. This blog explores the concept of straightness in GD&T, discusses straightness measurement, and clarifies the confusing topic of straightness versus flatness.

Understanding Straightness in GD&T and Methods of Measurement

What Is Straightness In Geometric Dimensioning and Tolerancing (GD&T)?

Straightness is a GD&T symbol that controls the straightness of a geometric entity. Straightness tolerance is utilized in various ways in engineering design. For instance, a table must be sufficiently straight (or flat—more on the technical difference later) to ensure that objects remain stable and do not roll off. Another example is the inner ring of a bearing. Its surface must be straight along the axial direction to ensure a proper fit on a shaft. In this case, any lack of straightness can lead to misalignment and damage to the bearing.

Why is GD&T Critical?

Engineers need to provide GD&T values along with the design. Without this information, manufacturers may use their internal standards, which could lead to issues when assembling parts with different tolerances. It is essential to clearly define a part’s acceptance criteria for quality control purposes. Please do so to ensure that the manufacturer can be held responsible for parts that meet tolerance requirements.

What Is Straightness In Geometric Dimensioning and Tolerancing (GD&T)

Straightness Symbol And Callout

Geometric Dimensioning and Tolerancing (GD&T) is a standardized way of defining tolerances in an engineering drawing. This section will discuss the straightness symbol and its callout in an engineering drawing.

The straightness symbol is represented by a flat, horizontal line (like a dash). The callout is referenced on either a surface or a feature of size. The figure below illustrates a simple example.

It’s important to note that in GD&T, straightness is not relative to a datum, which is required for other symbols like parallelism.

GD&T Symbol Classification

When adding GD&T (Geometric Dimensionalaling and Tolerancing) to a drawing, it is important to use standardized symbols and methodologies. Most CAD packages already have these symbols built in. The feature control frame specifies the type of tolerance, datum feature, and tolerance value.

There are different types of geometric tolerances, each with its explanation:

Orientation Tolerance: Refers to the shape of a feature based on a datum. All orientation tolerances require a datum.

Form Tolerance: Defines deviations from the ideal shape of a feature and does not need a datum.

Run-Out Tolerance: Defines the permissible deviation when a feature is rotated around its centerline. All run-out tolerances require a datum.

Profile Tolerance: Defines acceptable deviations of a profile line or plane. Profile tolerances do not require a datum.

Location Tolerance defines the position of a feature in relation to a datum. It is typically used to define the positioning of holes. All location tolerances require a datum.

GD&T Symbol Classification

Surface Straightness In GD&T

Surface straightness is commonly used to measure the flatness of cylindrical or flat surfaces. It establishes a tolerance area for upper and lower variations along a 2D line on the surface. For flat surfaces, the 2D line could be located anywhere on the surface, while for cylindrical surfaces, it is parallel to the axis. It’s important to note that surface straightness in GD&T is applied to the reference surface directly rather than size features.

How to Measure Surface Straightness?

Verifying surface straightness tolerance requires precision equipment and skilled personnel. Two main methods are used for measuring surface straightness.

Dial Gauge

Dial or height gauges are the standard equipment for manually measuring straightness. After securely fastening the gauge, the gauge’s contact point should be placed against the surface, and then it should be slid across the desired 2D line for measurement.

It’s crucial to ensure that the dial is calibrated and set to zero and that the plunger is barely pressed before the measure begins. Following these standard operating procedures guarantees precise measurement.

If the dial’s variation across the entire measurement length falls within the defined tolerance limits, the part meets the quality control standards.

Coordinate Measuring Machine (CMM)

Another method for performing straightness measurement involves using a CMM. This method provides more precise results than dial gauges. It is an automated process that requires CMM programming.

After mounting the piece on the CMM’s bed, the measurement probe scans the surface along the reference line and generates a report indicating if the part is within tolerance.

How to Measure Surface Straightness?

Straightness On Derived Median Line In Geometric Dimensioning and Tolerancing (GD&T)

The Derived Medial Line (DML) is an imaginary axis representing a cylindrical part’s symmetrical features. When a straightness tolerance is applied to the DML, the axis is perfectly straight within a cylindrical tolerance zone. This helps to detect any twists, bends, or other irregularities in the part’s shape.

It’s important to note that the tolerance zone is a 3D cylinder surrounding the true axis of the feature. Unlike some other geometric tolerance symbols in GD&T, the straightness symbol doesn’t require a datum, a reference point for measurement. For example, the perpendicularity symbol needs a datum to measure the perpendicularity of an edge, but the straightness symbol does not.

MMC/LMC Callout

The straightness modifier in GD&T has a special case when dealing with maximum material condition (Ⓜ) in DML cases. The maximum material condition (MMC) modifier ensures that a part doesn’t interfere with its mating components, even in its worst condition. It occurs when the part has the maximum material due to inaccuracies. In the case of shafts, the MMC happens when the shaft is at its maximum possible dimensions. At this size, the entire shaft must fit inside a hole with an equivalent diameter to the MMC. The formula connecting MMC, straightness tolerance, and gauge hole size can be applied to a specific straightness issue in GD&T with the MMC callout. Using the formula and reference values from the GD&T callout, we can determine a cylinder gauge size of 10.08, representing the maximum allowable deviation in the part. It comprises 0.05 from the size tolerance and 0.03 from the straightness.

Bonus Tolerance

It’s crucial to understand the concept of bonus tolerance. Bonus tolerance comes into play when a part’s size is not at its maximum limit. For example, suppose a part’s diameter is less than 10.05 (the maximum allowed). In that case, the straightness tolerance can be increased by the amount the diameter is less than its maximum value.

So, if the shaft size is 10.03, the bonus tolerance will be 0.02 (10.05 – 10.03). This amount is added to the straightness tolerance, making it 0.05 (0.03 + 0.02).

While this concept is advanced and requires a deep understanding of MMC/LMC and GD&T, this simple example illustrates how straightness behaves when used to control a DML along with MMC callout.

How to Measure the Straightness of a Derived Median Line?

A standard method uses a cylindrical gauge with a hole that acts as a quality-checking device to measure the straightness of derived median line features. If the piece easily fits inside the hole with minimal interference at any point, it passes the quality check. Alternatively, engineers can use a CMM to sample the piece’s center at different positions along the axial direction. However, this approach is time-consuming and only useful in a few cases. Generally, using a cylindrical gauge to check if the part is functional is a good guarantee of its quality.

What are The Differences Between Straightness and Flatness?

Designers, production engineers, and operators often confuse flatness and straightness in GD&T, but they are different concepts. Flatness applies to entire surfaces, while straightness controls only a line. Another way to understand the comparison is that flatness applies to 3D objects, while straightness is 2D. The flatness symbol is used for flat surfaces, where the entire surface must lie within the tolerance zone. Straightness applies to lines, edges, axes, and centerlines, giving designers more flexibility.


Straightness is a crucial aspect of GD&T in engineering. It clearly communicates design intent and tolerance requirements to factories and applies to various geometric features.

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